This is in the guide as November 1986, making it the first 8A in the country. But the excerpt from the new routes book (see reference in Leach's ascent) says it was done on Mother's Day, which is in March. UKC gives the date as 87.
It's in the 1986 stoney new routes book (page 192 of Phil Kelly's reproduction), and the Mother's Day part seems fairly conclusive so I think the date is 9th March 1986.
Peak Rock has a colour repro of the new routes book. It looks like 'Father's Day to Mothers Day' was written by someone else. He also says he tried it first at Easter 1986, then through the summer; so November 1986 sounds right to me. Other interesting notes: there was a pebble that he didn't want to stand on in case it broke, so he had a really hard sequence. Then he stood on it and did the route first time. Ben Moon used the pebble too. Not clear if the pebble still exists.
Good detective work 🕵️ hadn't spotted the father's day to mothers day section was written by someone else
I guess the other thing which might give some clues is the dates of other stuff nearby in the new routes book. I.e. if there's loads of stuff in the same part of the book recorded around November then that'd indicate it was also likely done around November.